An arc furnace of this type is generally constituted by a hearth made of very thick refractory material, thinner metal side walls with water circulation, and by a metal roof cooled by water circulation the central part of which, surrounding the electrodes is, however, made of very thick refractory material.
With furnaces of a model which is already old, constructed entirely of refractory material, it is now usual to control the furnace using statistical thermal mappings made with records obtained by means of radiation sensors, or transducers, positioned on the refractory wall of the furnace, above the maximum level of liquid steel therein. However, this type of transducer, termed a "wall probe", does not provide a satisfactory industrial solution because it is difficult to maintain and operate. With such furnaces, it is impossible to position these sensors in the roof because, as it is constructed of refractory bricks undergoing continual wear, this does not constitute a sufficiently rigid and unvarying support to receive such transducers.
In the case of modern furnaces comprising metal side walls cooled by water circulation and a partly metallic roof also cooled by water circulation, the attempt was obviously made, as formerly, to position heat flow transducers in the inner surface of the metal side walls. This method of construction, however, raises some problems. In practice, as was also the case previously with probes in refractory walls, these transducers rapidly becomes covered to a thickness of slag forming a lining, which lessens their sensitivity and, on account particularly of the "stair" formed at the junction of the side wall and the hearth as a result of their different thicknesses, a quite considerable bank of unmelted scrap iron forms at the edges of the furnace, eventually practically covering the walls to such a height that the wall probes are covered and are then unable to supply exact and immediate data on the thermal state of the furnace.